1. Field of the Invention
This invention concerns the cutting of screw-threads and more particularly relates to a thread-cutting tool and methods of cutting threads on both cylindrical and tapering surfaces.
2. Description of the prior art
The cutting of screw-threads in cylindrical surfaces presents problems, more particularly where the surface is of large diameter The conventional method of cutting such threads is to rotate the work-piece and the thread-cutting tool relative to one another about the axis of the surface and to feed the cutting tool axially along the surface. The teeth of the cutter are graded in size and shape along the length of the cutter so that the leading tooth takes a small cut off the metal and each succeeding tooth in the line cuts off a little more metal than the preceding tooth. In this way each turn of the screw-thread is formed gradually. In most cases, and for various reasons, a single cutter cannot be made long enough to bring the teeth of the screw-thread to their final profile, and it is usual to have a set of three or more taps or dies which are used in succession, so that the profile of the screw-thread is not complete until the final cutting tooth of the final tap or die of the set has passed along the full length of the screw-thread being cut. Where the screw-thread is to be formed on a large-diameter surface, the cost of a set of taps or dies is formidable and where a non-standard thread profile would be ideal may be prohibitive. To some extent the problem can be alleviated by forming the cutting teeth on an insert which is removably mounted on a carrier and which can readily be replaced when it becomes worn, but the teeth of the insert are absolutely essentially of the same form as those of a tap or die, that is to say that the shape of the teeth in the line is graded in size and shape, each tooth cutting a little more metal off than the preceding tooth, so that only when the thread has been traversed by the final tooth of the final insert of a set does the thread achieve its final profile. Thus whether an insert or a tap or die is used in this method, the teeth must be carefully graduated in size and shape and individually profiled to bring the screw-thread profile to its correct shape.